The present invention relates to an extruder provided with mixing pins.
As is known, screw extruders are generally composed of an extruder body with a cylindrical chamber containing, longitudinally, a helical screw which, when rotated, pushes, mixes and compresses the extrusion material towards an extrusion head on which a die is mounted.
According to the prior art it is also envisaged, in certain cases, that the cylindrical chamber contains mixing pins or rods. As can be seen from FIGS. 1A and 1B, which show an extrusion cylinder according to the prior art, the pins are normally arranged at regular intervals in the longitudinal direction and inserted radially in the casing of the extruder body.
In order to prevent interference during operation, the pins are fit in the casing walls, opposite interruptions in the thread of the extrusion screw, and also project inwardly the cylindrical chamber by a distance which is undoubtedly less than the transverse dimension of the gap between the core of the screw and the internal surface of the said chamber.
The main functions of the pins is that of creating an obstacle to the feeding movement of the extrusion material, which is thus disturbed and mixed more thoroughly, and of exchanging heat with extrusion material.
However, at least two types of problem are associated with this configuration.
A first problem is linked with the impossibility of correctly regulating the temperature of these pins.
U.S. Pat. No. 5,324,108 discloses a pin cylinder extruder wherein some pins are provided tangentially in the chamber. Such pins have an internal dead hole where thermal fluid is made to circulate back and fort. Such a kind of circulation path for the thermal fluid is not efficient and requires that the internal diameter of the pins be sufficiently large to accomodate an exit tube.
Moreover, in the event of one or more of the rods breaking, the fragments which break off end up inside the cylindrical chamber, contaminating the extrusion material and, in the worst instance, damaging the extrusion screw or the actual cylindrical chamber.
U.S. Pat. No. 5,480,227 discloses a screw extruder provided with some diagonally inclined pins. The wall of the cylinder receiving the screw has an anular recess. Pins are located very close to a bead of the screw and partially embedded in said cylinder wall. In this way, the extruded material may come into contact with a small portion of the lateral surface of the pins, resulting in a poor thermal exchange. Moreover, extruded material is chennelled between the pins but not actually mixed.
The object of the present invention is to overcome the problems encountered in the prior art by providing a new and original design of extruder in which the temperature of the pins can be easily controlled even with small diameters, the extrusion material may be intimately mixed and thermally regulated by the pins and it is possible to prevent damage resulting from possible breakage of these pins.
These objects are achieved by means of a screw extruder as described in claim 1.